The present invention relates to a dryer, particularly for granular or powdered products, which includes:
double-walled casing, the inner wall of which defines a cylindrical drying chamber for the aforesaid products and the outer and inner walls of which define between them an interspace for the circulation of a diathermic fluid, and PA1 a shaft mounted in the chamber and having means for enabling it to rotate about its own axis, at least one blade being securely fixed to the shaft and operable, on rotation of the latter, to exert a mixing action on the products and to detach them from the inner wall of the casing.
In prior art dryers of the above type the shaft is mounted so that it projects into the drying chamber with its axis coincident with that of the chamber.
Since the blades fixed to the shaft must extend close to the inner wall of the casing to carry out an effective scraping action, the overall diameter of the rotary members is substantially the same as the inner diameter of the drying chamber.
Hence, if the latter is rather large, the rotary members must be large and heavy, thus placing limitations on the performance of the dryer which, in some cases, are quite considerable.
For example, the speed of rotation of the shaft must not exceed certain values in order to avoid causing excessive twisting and bending stresses, thus limiting the effectiveness of the mixing.
In addition, significant clearance must be maintained between the facing surfaces of the distal ends of the blades and the inner wall of the chamber to prevent the machine from seizing up as a result of heat expansion or deformation caused by mechanical stress on the rotary members.
As a result, a significant layer of the material to be dried builds up on the inner wall of the chamber as the blades are unable to detach it and this behaves like a layer of insulation towards the diathermic fluid circulating in the interspace, thereby compromising the efficiency of the heat exchange in the dryer to a certain extent.